my best friend is on this and LOVES it. it made her kinda jittery for the first few days (this repeats when she changes doses). in fact, she was WILD the first 2 days. after the spazzies went away, she's found the meds to be nice and smooth. (lemme tell ya, they work, too, she's a whole new her! in a good way.)
she has lost tons of weight lately (really, a lot!) and didn't want to gain any on the meds. Effexor shouldn't make you gain weight... in fact weight loss is a side effect. (!)
Elavil is a tricyclic antidepressant and many of them make you crave carbs and sweets which usually results in a weight gain. i'm on one of these to promote deep sleep (really low doses). i told my doctor i was eating all the time and i didn't like it- she said "it's the medicine."
anyway, good luck. let me know how the Effexor works on the IBS if you try it.... if i need to change meds, that's the next one i want to try! (my best friend doesn't have IBS, so no help there.)